Placental perfusate comprises a collection of placental cells obtained by passage of a perfusion solution through the placental vasculature, and collection of the perfusion fluid from the vasculature, from the maternal surface of the placenta, or both. Methods of perfusing mammalian placentas are described, e.g., in U.S. Pat. No. 7,045,146 and U.S. Pat. No. 7,255,879. The population of placental cells obtained by perfusion is heterogeneous, comprising hematopoietic (CD34+) cells, nucleated cells such as granulocytes, monocytes and macrophages, a small percentage (less than 1%) tissue culture substrate-adherent placental stem cells, and natural killer cells. No one to date has described the use of placental perfusate, or populations of placental cells from perfusate, in the suppression of tumor cell proliferation.
Natural killer (NK) cells are cytotoxic lymphocytes that constitute a major component of the innate immune system. NK cells do not express T-cell antigen receptors (TCR), CD3 or surface immunoglobulins (Ig) B cell receptor, but usually express the surface markers CD16 (FcγRIII) and CD56 in humans. NK cells are cytotoxic; small granules in their cytoplasm contain special proteins such as perforin and proteases known as granzymes. Upon release in close proximity to a cell slated for killing, perforin forms pores in the cell membrane of the target cell through which the granzymes and associated molecules can enter, inducing apoptosis. One granzyme, granzyme B (also known as granzyme 2 and cytotoxic T-lymphocyte-associated serine esterase 1), is a serine protease crucial for rapid induction of target cell apoptosis in the cell-mediated immune response.
NK cells are activated in response to interferons or macrophage-derived cytokines. Activated NK cells are referred to as lymphokine activated killer (LAK) cells. NK cells possess two types of surface receptors, labeled “activating receptors” and “inhibitory receptors,” that control the cells' cytotoxic activity.
Among other activities, NK cells play a role in the host rejection of tumors. Because cancer cells have reduced or no class I MHC expression, they can become targets of NK cells. Accumulating clinical data suggest that haploidentical transplantation of human NK cells isolated from PBMC or bone marrow mediate potent anti-leukemia effects without possessing detectable graft-versus-host disease (GVHD). See Ruggeri et al., Science 295:2097-2100 (2002)). Natural killer cells can become activated by cells lacking, or displaying reduced levels of, major histocompatibility complex (MHC) proteins. Activated and expanded NK cells and LAK cells have been used in both ex vivo therapy and in vivo treatment of patients having advanced cancer, with some success against bone marrow related diseases, such as leukemia; breast cancer; and certain types of lymphoma. LAK cell treatment requires that the patient first receive IL-2, followed by leukopheresis and then an ex vivo incubation and culture of the harvested autologous blood cells in the presence of IL-2 for a few days. The LAK cells must be reinfused along with relatively high doses of IL-2 to complete the therapy. This purging treatment is expensive and can cause serious side effects. These include fluid retention, pulmonary edema, drop in blood pressure, and high fever.
In spite of the advantageous properties of NK cells in killing tumor cells and virus-infected cells, they remain difficult to work with and to apply in immunotherapy, primarily due to the difficulty in maintaining their tumor-targeting and tumoricidal capabilities during culture and expansion. Thus, there is a need in the art for a ready supply of natural killer cells.